Inhibition of rat and sheep adipose-tissue lipogenesis by palmitic acid and linoleic acid added in vitro.

Dietary polyunsaturated fatty acids are more potent inhibitors of rodent hepatic and adipose-tissue lipogenesis than are dietary saturated fatty acids (see Bartley &Abraham, 1972; Du & Kruger, 1972; Demeyer et al., 1974). The mechanism bywhich dietarypolyunsaturated fatty acids exert their extra-inhibitory effect is unknown, but there is the possibility that polyunsaturated fatty acids or their acyl-CoA esters are more potent inhibitors of one or more of the enzymes of lipogenesis than are saturated fatty acids or their acyl-CoA esters. In this study, the effect of adding various fatty acids to the incubation medium on the rate of incorporation of [1J4C]acetate into fatty acids by adipose-tissue slices has been compared. Rat adipose-tissue slices were incubated in Krebs-Ringer bicarbonate buffer (calcium concentration 1.27m~) containing 2m~-[l-'~C]acetate, 5m~-glucose and 40mg of albumin/ml. Fatty acids bound to albumin were added to give a final concentration of 1 mM. Tissue slices were incubated for 2 h at 37°C after which the lipids were extracted and the radioactivity of the fatty acids was determined as described previously (Vernon, 1975). Addition of either 1m~-palmitic acid or -1inoleic acid to the incubation medium decreased the rate of lipogenesis in rat parametrial adipose-tissue slices significantly (P < 0.05) to 52.0 f 8.0 and 56.7 f 11.4% of the rate observed in the absence of added fatty acids respectively; results are means ~ s . E . M . of six observations. Addition of insulin (0.1 i.u./ml) prevented the inhibition caused by adding fatty acids. The effects of palmitic acid and linoleic acid on the rate of lipogenesis in rat adiposetissue slices were dependent on concentration and were the same over the range 0.1-1 m ~ . The rate of uptake of palmitic acid and linoleic acid by rat adipose-tissue slices was obtained by measuring the rate of incorporation of [14C]palmitic acid and [14C]linoleic acid into tissue lipids. The rate of uptake of both fatty acids was identical and was linear with respect to time over 3 h. In view of current interest in increasing the amounts of polyunsaturated fatty acids in ruminant tissues, analogous experiments were performed with sheep perirenal adiposetissue slices. Again both palmitic acid and linoleic acid were found to result in an approx. 50% decrease in the rate of lipogenesis. However, addition of insulin did not prevent the inhibition of lipogenesis resulting from the addition of fatty acids to the incubation medium. The results show that, although both palmitic acid and linoleic acid inhibit the rate of both rat and sheep adipose-tissue lipogenesis when added in vitro, their effects are identical. Thus the different rates of lipogenesis observed after feeding polyunsaturated and saturated fatty acids do not appear to be due to polyunsaturated fatty acids or their acyl-CoA esters being more potent inhibitors of the activities of one or more enzymes of lipogenesis than saturated fatty acids on their acyl-CoA esters.